-LRB- CNN -RRB- -- Slipknot bassist Paul Gray 's death two months ago was from an accidental morphine overdose , the Polk County , Iowa , medical examiner said Monday .

Gray , 38 , was found dead by staff at a hotel near Des Moines , Iowa on May 24 , said Sgt. Dave Disney of the Urbandale Police Department .

Toxicology test results showed a fatal level of morphine and fentanyl -- a synthetic morphine -- in Gray 's body , said Dr. Gregory Schmunk .

Schmunk also noted the presence of significant heart disease , according to a police statement Monday .

The autopsy did show that Gray had been dead for several hours before his body was found , Schmunk said .

Grammy-winning metal band Slipknot -- whose fans are affectionately known as `` maggots '' -- was formed in Des Moines in late 1995 and became immediately recognizable for the dark and often grotesque homemade Halloween masks its nine members wore on stage . The band won a Grammy award in 2006 for best metal performance for `` Before I Forget . ''

Slipknot 's self-titled debut album went platinum in 2000 . Its fourth album , `` All Hope is Gone , '' spent 69 weeks on Billboard 's Top 200 charts , debuting at No. 1 its first week in 2008 .

Gray was arrested in Iowa in 2003 on drug possession charges when police found small amounts of marijuana , two syringes and a substance that field-tested positive for cocaine after he was involved in a wreck with another motorist .

CNN 's Denise Quan contributed to this report .

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Autopsy showed morphine overdose killed Gray

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Slipknot bassist was found dead May 24

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Gray also sufffered `` significant heart disease ''